1. Field of the Invention
The present invention relates to a laser beam optical scanning device for writing data on an optical recording medium preferably for use in a variety of devices including a digital copying machine, a laser printer, an optical disk, and, in particular, to an optical scanning device that is capable of reducing variations in shading and increase light usage during a scanning process.
2. Description of the Related Art
An optical scanning device for scanning a scanning surface aligned with the surface of an optical recording medium, such as a photosensitive material, is well known in relation to laser printers or the like. In an optical scanning device, a usual optical arrangement is such that a laser beam from a laser beam source is deflected by a light deflector, such as a rotating polygon mirror, and applied to the scanning surface as a light spot by a scanning lens. Thus, the incident angles of the laser beam on the reflective surface of the light deflector and on the scanning lens are caused to continuously vary during line scanning in the main scanning direction.
Reflectance of the deflecting reflective surface and reflectance and transmittance of the surface of the scanning lens vary in correspondence with the respective incident angles, so that the intensity of the light spot on the scanning surface generally fluctuates with an image height in the main scanning direction, resulting in an unevenness in image density in the line scanning or a deterioration in gradation. This phenomenon, which is referred to as xe2x80x9cshadingxe2x80x9d, is serious when the direction of polarization of the laser beam impinging upon the deflecting reflective surface is parallel with or perpendicular to the deflecting direction. Generally, the light intensity tends to be reduced or increased at either end in the main scanning direction with respect to the central image height in the main scanning direction.
To overcome this problem, a filter which has a transmittance distribution has conventionally been used in such an optical system to compensate for the variation in the intensity of light of the scanning line. However, there are limitations to the acceptable size and position of the filter. Further, use of such a filter results in an increase in cost of the optical scanning apparatus.
Japanese Patent Laid-Open No. 5-303049 proposed a construction in which a xc2xc wavelength plate is arranged in the optical path between the light source and the light deflector. Due to this construction, the polarization of the beam impinging upon the light deflector is converted to circularly polarized light, and reflectance of the reflective surface of the light deflector is kept substantially constant within the deflecting region, whereby a reduction in shading is realized.
However, the above-described construction uses a xc2xc wavelength plate, which is expensive, resulting in an increase in the production cost of the optical scanning device itself.
To overcome the problems described above, the preferred embodiments of the present invention provide an optical scanning device having a very simple construction which significantly reduces shading, and significantly improves reflectance and light usage.
In accordance with a preferred embodiment of the present invention, an optical scanning device includes a light source, which generates a laser beam, a light deflector for deflecting the laser beam in a light deflection direction, and a scanning lens for focusing the deflected laser beam at a spot on a scanning surface to thereby perform optical scanning, wherein the light source, the deflector, and the scanning lens are located along an optical axis, and the light source is tilted relative to the optical axis by an angle of about 17.5xc2x0 to about 27.5xc2x0 or by an angle of about 62.5xc2x0 to 72.5xc2x0, and generates the laser beam, such that the laser beam, which is impinged on the reflective surface, is light polarized in a direction between a direction that is parallel to the light deflection direction and a direction that is perpendicular to the light deflection direction.
Thus, instead of using an arrangement of a light source which produces only a P-polarized light or only an S-polarized light, the preferred embodiments of the present invention arrange a light source relative to an optical axis so as to produce a laser beam which includes a combination of P-polarized light and S-polarized light and minimizes the effect of a plurality of disadvantageous conditions. As will be described below, this arrangement of the light source and the resulting laser beam including a combination of P-polarized light and S-polarized light significantly minimizes shading and reduces variations in light intensity, while also maximizing light usage, during a scanning operation.
The above-described preferred embodiment preferably includes an aperture located between the light source and the deflector. The aperture preferably has a length that is larger than a width, where the length extends in a direction of a major axis of an ellipsoid shaped light beam output by the light source at a location of the aperture. Also, the aperture in this preferred embodiment preferably has a substantially rectangular shape or may have a substantially square shape.
In order to further maximize light usage, as described in more detail below, the corners of the aperture may be cut so as to define oblique angles relative to sides of the aperture.
According to another preferred embodiment, an optical scanning apparatus includes a light source generating a laser beam, a deflector having a reflective surface arranged relative to the light source to deflect the laser beam via the reflective surface in a light deflection direction, a scanning lens arranged relative to the deflector to focus the deflected laser beam at a spot on a scanning surface to thereby perform optical scanning and an aperture located between the light source and the deflector, the aperture having a substantially square shape, wherein the light source, the deflector and the scanning lens are located along an optical axis and the light source is tilted relative to the optical axis by an angle of about 45xc2x0, and generates the laser beam such that the laser beam which is impinged on the reflective surface is light polarized in a direction between a direction that is parallel to the light deflection direction and a direction that is perpendicular to the light deflection direction.
As with the previously described preferred embodiment, this arrangement also significantly minimizes shading and reduces variations in light intensity, while maximizing light usage, during a scanning operation.
The arrangement of the light source and the apertures described with respect to the above preferred embodiments achieves an excellent combination of elements and arrangement thereof which provides an extremely improved laser beam including P-polarized light and S-polarized light to produce the significant reduction in shading and variation in light intensity, while maximizing light usage.
The above-mentioned light source may be an edge-emitting type laser diode. In addition, the light source may have an array structure including a plurality of light emitting sections each disposed on a common substrate and capable of independently effecting optical modulation.
These and other features, advantages and elements of the present invention will be apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings as described below.